The expansion of the drilling industry to offshore locations has led to the development of several types of drilling units. These consist of submersible drilling platforms, jack-up drilling platforms and two major classes of floating vessels. The two major classes of floating drilling vessels are those of conventional hull form or drill ships and those of semi-submersible or column stabilized units.
The drill ship is an adaptation fo a standard seagoing ship of mono-hull form with the addition of a substructure with a moon pool and/or cantilevers from which the drilling operations may be carried out. These vessels are also equipped with some additional means of positioning the unit over the drill center so that the vessel will maintain a close relationship with the bore hole in the seabed. These vessels may be held in position by either a mooring system or a dynamic positioning system. It is well known that ship type drilling units are very susceptible to wave action and will tend to move in a direct relationship with the sea state encountered. Since the vessel is connected to the seabed by a riser and the drill string is in contact with the bottom of the bore hole, motions of the vessel with respect to the seabed are extremely important to be able to maintain the drilling posture.
In response to the need for development of offshore petroleum exploration and development in increasingly more hostile and deeper water, the significance of a vessel with improved motions with respect to the seabed became apparent. For this purpose, industry has adopted and used a series of semi-submerged or semi-submersible drilling units. In essence, all of these semi-submersible drilling units or vessels comprise a wide base and totally submerged pontoons or mats. A series of vertical buoyant columns rise from this submerged base supporting a horizontal deck or platform which is maintained well above the normal expected wave crests. Upon this platform or deck, the living quarters, machinery spaces and drilling package are located. The drilling center usually consists of a cellar deck or storage area for the subsea equipment, a moon pool through which the drilling operation is carried out which is usually located in the cellar deck and a substructure which is mounted above the cellar deck area upon which the draw works, rotary and derrick are mounted. Adjacent to this area is a pipe rack area for the storage of the marine riser, drill pipe, drill collars, casing and other tubular products. The semi-submersible drilling unit is also maintained in position against the forces of the environment by use of either a fixed mooring system or an active propulsion system (dynamic positioning) a combination of the two (thruster-assisted mooring). In either case, the semi-submersible is still supported on the ocean surface by its own buoyant effect and is also suspectible to wave induced motion.
There have been recent developments in rotatable and swivable drilling units of the ship type to reduce the sensitivity of the unit to the roll and pitch motions and thereby improve their motion characteristics, thus reducing their down time for weather. These developments, however, have done little or nothing for the improvement of the heave or vertical motions of the platform with respect to the ocean floor. Many new devices have been introduced, such as motion compensators, riser tensioners and guideline tensioners which are active devices and greatly reduce the effect of the heave motion of the unit for the drilling operation. These items, however, are of a mechanical nature and do result in down time due to maintenance and also have their limits with respect to the range of sea states which they can effectively dampen.
The generally recognized design of semi-submersible platforms for minimizing the sensitivity of the unit to wave induced motions is known to consist of a lower hull or a group of pontoons upon which are deployed any number of buoyant columns arranged such that their collective water plane areas are spread significantly to provide a stable platform. The buoyancy for the unit is provided by the displacement of the lower hull or hulls and the vertical columns of the unit below the waterline. The water plane area of these vertical columns, the effective cross-sectional area of the columns at the lever of the waterline, is known to be a significant design factor for both minimizing the wave motion sensitivity and providing a stable platform with significant load carrying-capability to allow the vessel to perform its intended function. It is a trade-off between these requirements for improved motion characteristics for better drilling operations and required water plane area for a stable platform that is normally the prime concern of a naval architect with respect to the design of a semi-submersible drilling unit.
The geometric configuration of the columns of a semi-submersible unit are somewhat determined by the vessel's intended service as well as by its designer's philosophy. With increased needs for semi-submersibles on a worldwide market, it became essential to have units which were more highly mobile thus the current gereration of twin hull semi-submersibles having four, six, and eight columns evolved. The lower hulls of these units are generally of a ship shape form and during transit the unit performs similar to a catamaran type vessel.
The current state of the art attempts to reduce the motion sensitivity of the semi-submersible unit, which includes varying the shape of the buoyant columns, placing active or passive hydrodynamic dampening devices upon or within the columns and by changing the geometrical shape of the pontoons. All of these items are effective to one degree or another, however, all have their respective shortcomings.